The present invention relates to suspension systems for snowmobiles, and more particularly, the present invention relates to snowmobile rear suspensions. In an illustrated embodiment, progressive rate rear suspension architecture for snowmobiles is disclosed.
Performance characteristics of snowmobiles, including the comfort of the ride, depend on a variety of systems and components, including the snowmobile suspension. Typically, a snowmobile suspension includes two systems, a front suspension system for a pair of skis and a rear suspension system for the track.
The rear suspension of a snowmobile supports an endless track driven by the snowmobile engine to propel the machine. The track is supported beneath a vehicle chassis by a suspension that is designed to provide a comfortable ride and to help absorb the shock of the snowmobile crossing uneven terrain. Most modern snowmobiles use a slide rail suspension which incorporates a pair of slide rails along with several idler wheels to support the track in its configuration. The slide rails are typically suspended beneath the chassis by a pair of suspension arms, with each arm being attached at its upper end to the chassis of the snowmobile, and at its lower end to the slide rails. The mechanical linkage of the slide rails to the suspension arms and to the snowmobile chassis typically is provided by springs and at least one element acting along a linear path, such as a shock absorber, damper, air shock, shock and spring combination, or other linear force element (LFE). The springs are loaded to bias the slide rails downwardly away from the snowmobile chassis and the shock absorbers; dampers or LFEs provide damping forces for ride comfort.
There are presently two general types of snowmobile rear suspensions in all of the snowmobile industry: coupled and uncoupled. The term “coupled” is given to suspensions that have dependant kinematics front-to-rear and/or rear-to-front (relative to the rear suspension rail beam). That is, a suspension is coupled “front-to-rear” when the front of the suspension is deflected vertically and the rear also moves vertically to some degree. A suspension is coupled “rear-to-front” when the rear of the suspension is deflected vertically and the front also moves vertically to some degree. A suspension is considered to be coupled “tighter” front-to-rear, or increased coupling bias to the front, if a front deflection causes near the same rear deflection. The same is true if a suspension is coupled “tighter” rear-to-front, or increased coupling bias to the rear: a rear deflection causes near the same front deflection. An uncoupled rear suspension functions independently front-to-rear and rear-to-front. A deflection of the front portion of the suspension causes little to no deflection of the rear portion and vice versa.
There are two main advantages to a coupled suspension. First, a coupled suspension shares rate when coupled. There is a distinct rate associated with the front of the suspension and a separate distinct rate associated with the rear of the suspension; when a suspension “couples” it borrows the rate of both the front and rear of the suspension so the overall rate becomes higher than could have been achieved without coupling. Second, coupling is used to control weight transfer during acceleration. An uncoupled suspension will allow excessive chassis pitch due to the independence of the suspension. Coupling stops this by limiting the angle of the slide rail and by increasing the rate of the suspension and “locking” the suspension geometry.
Typically the use of a coupled suspension, uncoupled suspension, and the degree to which a suspension is coupled depends on the expected use. Coupled suspensions are mostly used on trail/performance snowmobiles where large bumps and tight corners require increased rate and controllable weight transfer. Uncoupled suspensions are used on deep snow/long track snowmobiles where weight transfer and traction are more important.
There are many ways to create a coupled rear suspension. The simplest form of a rear suspension is a four-link suspension created by the chassis, two arms, and the slide rails all connected with rotational pivots. This type of suspension yields only one degree of freedom. The slide rail motion and suspension kinematics are predefined by the length of the 4 links and cannot be altered due to the location of the input (front, rear, or between). This is not desirable because the slide rail will not follow undulating terrain or allow any angle change relative to the chassis due to acceleration. To fix this problem with a basic four-link, one of the links is allowed to change length to some degree. The geometry of the four-link therefore changes relative to the location of the input. A deflection of the front portion of the suspension yields one distinct four-link geometry and a deflection of the rear portion of the suspension yields different distinct four-link geometry. There is always some degree of uncoupled behavior in a coupled suspension when the geometry is not locked front-to-rear or rear-to-front. It is important to note that most coupling is focused on rear-to-front to help control excessive weight transfer. The majority of differences in rear suspension architecture are driven by accomplishing this same goal of a “sloppy” four-link in different ways.
FIG. 1 illustrates an example of a traditional rear suspension 10 (illustratively a 2D model of the Polaris Fusion® snowmobile rear suspension design) having slide rails 12, a front suspension arm 14 and a rear suspension arm 16. Front suspension arm 14 is coupled to the slide rails 12 by pivot connection 18. An opposite end of front suspension arm 14 is pivotably coupled to the chassis. Rear suspension arm 16 is pivotably coupled to the slide rails 12 by pivot connection 20. An opposite end of the rear suspension arm 16 is pivotably coupled to the chassis. Torsion springs are illustratively mounted between the rear torque arm and slide rails 12. First and second linear force elements (LFE) 22 and 24 are coupled between the first and second suspension arms 14 and 16, respectively and the slide rails 12 in a conventional manner.
FIG. 1 labels the following geometry of a four bar link which is similar in most snowmobile rear suspensions as illustrated by the lines defining: A) Front Link, B) Rear Link, C) Rail Link, and D) Chassis Link. These links and their relative lengths govern the majority of rear suspension kinematics.
The coupling bias behavior as described above is dependant on this four-link geometry and is important to rear suspension rate, impact harshness, and ride quality. For example, a perfectly symmetric four-link (A=B and C=D, A parallel to B and C parallel to D) will yield a rail angle that is maintained at the same angle throughout travel. In other words, the rail 12 does not rotate relative to the chassis as the suspension is compressed. This type of movement is not desirable due to the need to achieve traction on undulating terrain. As deviations to this symmetric geometry are made, the rail angle will change throughout suspension travel.
As traditional suspensions are compressed, the front arm begins to “point” at the rear arm mount location. This is known as “over centering”. FIGS. 2 and 3 illustrate this graphically, showing how links A and C have become substantially a straight line.
A rear suspension that is coupled rear-to-front has the same over-centering problems as discussed above for a front load situation, but to a larger degree. FIG. 4 illustrates this problem graphically when a rear load is applied as illustrated by arrow 27, showing how link B has crossed over link C. As mentioned above, over-centering drastically reduces effective suspension rate and damper velocities.
The problem lies in packaging a four link geometry that does not move over-center during compression. Consider another common four-link suspension, the SLA (Short-Long Arm) suspension. FIG. 5 illustrates a common snowmobile SLA front suspension 30 with labels equivalent to FIG. 1 for links A, B, C and D. Details of the SLA front suspension are described in U.S. Pat. No. 6,942,050 which is incorporated herein by reference. This arrangement does not move over-center upon compression due the placement of chassis mount points of A and B at locations 17 and 19, respectively, in FIG. 5. The vertical separation is a high percentage of link D. Therefore, the first factor in eliminating over-centering in rear suspensions is to place the rear arm significantly higher than the front arm as outlined in the above discussion about coupling behavior.
Simply moving the rear point of a conventional suspension upward is not feasible. The rear arm needs to become significantly shorter than the front. Typical link ratios (NB) on conventional suspensions are between 1 to 1.5. Ratios other than this are not feasible or do not package in current design envelopes. However, to accommodate a higher rear mount, A/B ratios need to increase to the range of about 1.6 to 2.0. Therefore, in an illustrated embodiment, A/B ratios are preferably 1.6 to 2.0 or greater in coupled suspensions. FIG. 6 illustrates the difference in a rear load case coupling angle between a conventional suspension labeled as “Prior Art” (illustratively the Polaris IQ 440 suspension) and the illustrated embodiment described below (labeled as “Improved Rear Suspension Coupled” and “Improved Rear Suspension Uncoupled”).
FIG. 7 illustrates the difference in the coupling angle between a conventional suspension and the suspension of the present improved suspension invention described below. Conventional suspensions yield a front coupling angle that increases through travel. This means that as the conventional suspension is compressed, the angle of the slide rail increases. This type of behavior is not ideal because as the rail angle increases, rate and damper velocities decrease ultimately resulting in a regressive suspension. More desirable is a rail angle that decreases as the suspension is compressed; thus, effectively making the suspension rate progressive (the more regressive the rail angle, the more progressive the rate). However, an increasing coupling angle is difficult to eliminate due to the packaging of a traditional snowmobile suspension. In the illustrated embodiment of the present invention, unconventional packaging of the suspension components results in a vertical difference between the front arm and rear arm chassis mounts of preferably 20% or more of the chassis link length (D) which results in a decreasing coupling angle.
Further examination of coupling behavior yields two constraints necessary to maintain reasonable component loads and basic function of the rail/ground interface. First, this angle should be positive. In other words, when a load is applied to the front of the suspension as illustrated by arrow 25 in FIG. 3, the front portion of the slide rails 12 moves more than the rear portion and vice versa for a load applied to the rear of the suspension. Second, there should be no inflections, or change in sign of the slope, in the curve of rail angle versus vertical deflection, as shown in FIGS. 6 and 7. In other words, when a load is applied to the front of the suspension, at no point should the rear of the suspension begin to move faster than the front and vice versa for a load applied to the rear of the suspension.
Because an uncoupled suspension does not form a distinct four-link, no over-centering can occur. No link ratio is then necessary for a rear load case in an uncoupled suspension. This is very beneficial, but excessive vehicle pitch and lack of vertical rate usually make uncoupled suspensions behave poorly for load carrying capacity and ride quality. Typically, for these suspensions a link ratio is then tuned only for the front load case. The shock/spring ratio can be tuned to help counteract the deficiencies of an uncoupled suspension. In this way, the rear arm geometry is tuned exclusively to maximize rear load case rate. Therefore, linkage arm length ratios are tuned for front coupling and rear rate in uncoupled suspensions.
As discussed above, the majority of snowmobile rear suspension architectures utilize a combination of springs, dampers, or other similar linear force elements (LFE), all packaged within the envelope of the track. Regardless of how these elements are packaged, these designs typically use two methods to generate vertical rate: 1) the LFE is located so that there is some vertical component reacted between the suspension arm and rail beam, and 2) the LFE is connected to the suspension arm such that a torque reaction is generated about the upper pivot. The inherent problem is that these designs lose rate near full jounce due to the suspension mechanism components becoming generally planer. That is, all the suspension components fold down until they are lying relatively flat as the suspension components move at full jounce. This is due to the large vertical travel requirements of a snowmobile suspension.
The result of the suspension components becoming planar is that the load vector of the LFEs begins to point horizontally instead of vertically. This transfers load into the internal components of the suspension and does not react vertically to suspend the vehicle. Also, as the suspension components become planar, the moment arm through which the suspension reacts increases at a faster rate than can be controlled by the shock/spring ratio, regardless of the type of linkage used to accelerate the shock/spring.
With reference again to FIGS. 1 and 2, FIG. 1 shows a 2D representation of suspension 10 at full rebound. FIG. 2 shows suspension 10 at full jounce. The front and rear LFEs 22, 24 become generally planar and lay down and point nearly horizontally in FIG. 2. The rear torque arms get “longer” measured from the upper pivot to lower pivot in the horizontal direction. Even with a complicated linkage to help stroke the rear LFE 24, a progressive rate cannot be maintained due to the two reasons listed above. This is true for all conventional snowmobile rear suspension systems.
Load at the slide rails and, more importantly, the bias between front and rear load is directly related to coupling, especially for a front load case. Consider the traditional suspension as illustrated in FIG. 1. The architecture is such that the front spring/damper 22 acts between the front arm 14 and the slide rail 12, and both the torsion springs and rear damper 24 act between the rear arm 16 and the slide rail 12 near the front. Therefore, during a front load case, both springs and dampers 22, 24 have a large effect on load and rate. The same is true of a rear load case. Attempting to tune the front LFE 22 will change the load/rate at the front and rear, and vice versa. Also if the coupling were increased, the rail angle decreases through travel and the rate will increase. In order to tune the suspension rate, the front LFE 22, rear LFE 24 and torsion springs, and coupling angle all need to be adjusted.
To improve this system: 1) Front coupling can be used primarily to control front load/rate, 2) Front preload is adjusted by a small LFE near the front of the rail (has a very small affect on rate), and 3) rear preload and rate is determined by the rear arm only. To achieve this with actual architecture, the main rear LFE needs to react only at the rear arm and with no other suspension components. Therefore reacting the LFE on the chassis in the above discussion is important not only for progressive rate, but also for load bias. When these three conditions are true, rear coupling does not greatly influence rate. This is realized because the front LFE is only used for preload so there is generally very little rate to “borrow” from the front of the rail during a rear load case.
Progressive rate suspensions have not yet been achieved in snowmobile rear suspension designs because 1) the vertical component of the LFE becomes very small as the LFEs become horizontal and planar with the suspension during jounce, and 2) the rotational component of the LFE about the arm pivot also cannot increase faster than the increase in arm length moment.
The state of snowmobile rear suspensions in the industry consists entirely of falling rate, or regressive suspension designs. Even though there is a large variety in the suspension architecture from one manufacture to another, commercially available designs yield an overall suspension stiffness that decreases as the suspension is compressed toward full jounce. Some architectures yield discontinuities that may locally spike the rate for a short time (such as an overload spring), but afterwards the rate continues to decrease. Because most design effort is directed at optimizing a damper or spring motion ratio instead of analyzing the entire suspension system there are currently no progressive rate suspensions in the industry.
Now with regard to chassis construction, traditional snowmobile chassis structures consist of elements common to each manufacturer, especially in the tunnel and rear suspension portion of the snowmobile. Typically, the rear suspension includes two suspension arms attached to the chassis tunnel frame and a drive shaft mounted forward of the front arm to drive the endless track.
This conventional suspension arrangement poses two problems. First, track tension through suspension travel relies on the relative placement of the suspension arms and wheels to the drive shaft. Suspension mount locations are often determined not only by specific, desired suspension characteristics, but also on track tension packaging. Problems are encountered from both an over and under tensioning track standpoint. Second, the front arm placement is limited to remain outside the drive sprocket diameter due to interference with drive train components. This creates problems when attempting to change the weight transfer behavior of the rear suspension, which is dominated by front arm mount location.
Achieving the mount points for desirable rate and kinematics is only half the challenge of snowmobile suspension design. Packaging a track around the suspension is the other. Traditional suspensions sacrifice more optimum suspension geometry to provide track tensioning and packaging which can be extremely difficult to manage.
All of these problems are solved by mounting the front swing arm coaxial with the drive shaft as discussed below. Because the front swing arm rotates around the same axis as the track drive sprocket, track tension is only influenced by the slide rail approach bend profile and a rear suspension idler pulley. Also, the coaxial placement of the arm creates improved weight transfer behavior of the rear suspension.
In order to generate necessary traction under acceleration, weight transfer and pitch need to be considered. Suspension parameters are tuned to facilitate the shift of vehicle weight from the skis to the track. This shift in weight is imperative for snowmobile acceleration due to slippery ground conditions. There are many parameters, but the two that dominate are front arm mount locations and carrier wheel.
Vehicle pitch is partially a result of this weight shift, but excessive pitch can result without increased traction. Packaging constraints, such as track carrier wheels, within the design of the suspension may limit or increase the ability of the vehicle to pitch.
With this design, the improved suspension may eliminate the carrier wheel. This changes the load vector into the suspension from the track due to tractive forces between the track and ground. In the illustrated embodiment, the load vector from the track is more horizontal which induces less pitch and weight transfer than a traditional suspension. To improve this, the front arm is moved significantly forward to facilitate weight transfer. This point can move forward incrementally until it encounters the drive wheel inscribed circle. At this point, it can only move coaxial with the drive sprocket. The illustrated embodiment of the present invention utilizes a coaxial front arm mount as discussed herein to facilitate weight transfer and pitch.
As for the frame assemblies, traditional snowmobiles utilize a long tunnel structure to which the driveshaft and rear suspension mounts beneath. Above the tunnel typically sits a fuel tank and seat. This type of structure is adequate because most spring/damper forces are reacted internal to the suspension and between the front and rear arm mounts. Additional structure to the base tunnel is only required between these mounts.